The present invention relates to laser micromachining and, in particular, to a method and apparatus employing an fast steering mirror to move a laser spot having a focused spot size in a desired pattern on a substrate to remove a target area that is larger than the focused spot size on the substrate.
The background is presented herein only by way of example to multilayer electronic work pieces, such as integrated-circuit chip packages, multichip modules (MCMs) and high-density interconnect circuit boards, that have become the most preferred components of the electronics packaging industry.
Devices for packaging single chips such as ball grid arrays, pin grid arrays, circuit boards, and hybrid microcircuits typically include separate component layers of metal and an organic dielectric and/or reinforcement materials, as well as other new materials. Much recent work has been directed toward developing laser-based micromachining techniques to form vias in, or otherwise process, these types of electronic materials. Vias are discussed herein only by way of example to micromachining and may take the form of complete through-holes or incomplete holes called blind vias. Unfortunately, laser micromachining encompasses numerous variables including laser types, operating costs, and laser- and target material-specific operating parameters such as beam wavelength, power, and spot size, such that the resulting machining throughputs and hole quality vary widely.
Pulsed ultraviolet (UV) lasers currently used in micromachining operations produce relatively small spot sizes compared to the kerf widths and hole diameters desired for many applications. Laser machining throughput for creation of such feature geometries that are large compared to the laser spot size, hereinafter referred to as xe2x80x9ccontoured machining,xe2x80x9d may be increased by employing a larger and lower power density laser beam. As described in U.S. Pat. No. 5,841,099, by operating the laser out of focus, Owen et al. can effectively enlarge the laser spot size and reduce its energy density. U.S. Pat. No. 5,593,606 and U.S. Pat. No. 5,841,099, both of Owen et al. describe advantages of employing UV laser systems to generate laser output pulses within advantageous parameters to form vias or blind vias in multilayer devices. These patents mention well known techniques in which vias having diameters larger than that of the focused spot size may be produced by trepanning, concentric circle processing, or spiral processing. These techniques will hereinafter be collectively referred to as xe2x80x9ccontoured drilling.xe2x80x9d
Unfortunately, operating the laser out of focus often results in unpredictable and undesirable energy distribution and spot shape and adversely impacts via quality, including the via wall taper, the degree of melting of the copper layer at the bottom of the via, and the height of the xe2x80x9crimxe2x80x9d around the periphery of the via caused by the splash of molten copper during drilling. Furthermore, because the spot size entering conventional collimating and focusing optics is inversely proportional to the spot size impacting the target, the power density applied to the optics quickly exceeds the damage threshold of the optics.
U.S. Pat. No. 4,461,947 of Ward discloses a method of contoured drilling in which a lens is rotated within a plane perpendicular to an incident laser beam to affect a target area that is greater in size than that of the focused laser spot. The lens rotation is independent of the position of the supporting mounting arm. Ward also discloses a prior art method of contoured drilling that relies on movement of the mounting arm within a plane to effect lens rotation. In the background, Ward discloses that the beam may be rotated by a rotating mirror.
U.S. Pat. No. 5,571,430 of Kawasaki et al. discloses a laser welding system that employs a concave condensing mirror that is pivotal about a first axis and supported by a rotary support member on a bearing such that the mirror is rotatable about a second axis perpendicular to the first axis. The mirror is oscillated about the first axis to increase the xe2x80x9cwidthxe2x80x9d of target removed and rotated about the second axis to create an annular pattern.
An object of the present invention is, therefore, to provide a method or apparatus for quickly spatially spreading out the focused laser spots, and therefore the energy density, of high repetition rate laser pulses.
Another object of the invention is rapidly create geometric features having dimensions greater than those of the focused laser spot.
A further object of the invention is to improve the throughput and/or quality of work pieces in such laser machining operations.
U.S. Pat. Nos. 5,751,585 and 5,847,960 of Cutler et al. and U.S. Pat. No. 6,430,465 B2 of Cutler include descriptions of split-axis positioning systems, in which the upper stage is not supported by, and moves independently from, the lower stage and in which the work piece is carried on one axis or stage while the tool is carried on the other axis or stage. These positioning systems have one or more upper stages, which each support a fast positioner, and can process one or multiple work pieces simultaneously at high throughput rates because the independently supported stages each carry less inertial mass and can accelerate, decelerate, or change direction more quickly than can those of a stacked stage system. Thus, because the mass of one stage is not carried on the other stage, the resonance frequencies for a given load are increased. Furthermore, the slow and fast positioners are adapted to move, without necessarily stopping, in response to a stream of positioning command data while coordinating their individually moving positions to produce temporarily stationary tool positions over target locations defined by the database. These split-axis, multirate positioning systems reduce the fast positioner movement range limitations of prior systems while providing significantly increased tool processing throughput and can work from panelized or unpanelized databases.
Although such split-axis positioning systems are becoming even more advantageous as the overall size and weight of the work pieces increase, utilizing longer and hence more massive stages, they may not provide sufficient bandwidth to effectively spread out the energy by large geometric spacing between the laser pulses at high pulse repetition frequencies (PRFs).
The present invention employs, therefore, an fast steering mirror, such as a piezoelectrically controlled mirror, in the beam path to continuously move the laser beam in a high speed prescribed pattern about a nominal target position to spatially separate the focused laser spots generated at a high laser repetition rate and thereby create geometric features having dimensions greater than those of the focused laser spot. The invention permits a series of laser pulses at a given repetition rate to appear as a series of larger diameter pulses at a lower pulse rate without the beam quality problems associated with working out of focus.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof which proceeds with reference to the accompanying drawings.